Substance Disorders, Substance Use Treatment, and Sexual Orientation in Youth Substance abuse and dependence are critical public health problems affecting the U.S. and they cause enormous financial and social burden, morbidity, and mortality. Sexual-minority populations (e.g., lesbian, gay, bisexual [LGB]) are disproportionately affected by substance use, substance use disorders, and co-occurring mental health problems. However, there are almost no empirical investigations among youth that would enable researchers and clinicians to: 1) understand patterns and causes of sexual-orientation disparities in risk for substance use disorders, and 2) elucidate sexual minorities' barriers to receiving needed substance use treatment and their treatment experiences. Consequently, huge knowledge gaps exist in understanding which subpopulations of sexual-minority youths are most at risk for substance use disorders, their barriers to receiving needed treatment, and effective treatment approaches. The recent Institute of Medicine (IOM) report, The Health of Lesbian, Gay, Bisexual, and Transgender People: Building a Foundation for Better Understanding, identifies large health disparities in the LGB population, extensive gaps in understanding LGB health, and methodological complexities in conducting LGB research. To address these problems, we will carry out a mixed methods study with the Growing Up Today Study, two ongoing longitudinal cohorts of 27,324 youth begun in 1996 and 2004. The proposed project will focus on a range of substance disorders including alcohol abuse and dependence, drug abuse and dependence, and nicotine dependence. In order to develop targeted, effective, evidence-based preventive and treatment interventions for sexual-minority youth, it is necessary first to: 1) understand the extent to which substance disorders disproportionately impact this population, 2) identify subpopulations at greatest risk, 3) understand causal mechanisms contributing to sexual-orientation-based disparities, 4) identify specific barriers sexual minoritis face receiving needed treatment, and 5) identify effective treatment strategies. Our study has high potential for public health impact, as we propose to contribute essential knowledge to all 5 of these critical areas, with a focus on the developmental periods of adolescence and emerging adulthood.